1+1 systems allow two subscriber terminals to be connected to a telephone exchange by a common two-wire link. One of the terminals is conventionally connected, using an audio telephone channel with two-way transmission over the base band on the common two-wire link. It is remote powered by the exchange battery via the common two-wire link. The other terminal is linked by carrier currents. Each transmission direction of its "super-audio" channel is transmitted over the common two-wire link via separate frequency-transposed bands. Said other terminal may likewise be remote supplied from the exchange battery via the common two-wire link. The super-audio transmissions are superimposed on the common two-wire link or separated therefrom, as the case may be at each end of said link by means of respective filter circuits, each of which comprises a low-pass filter connecting the common two-wire link to an audio-only, two wire line, and two band pass filters connecting the common two-wire link to a four-wire line on which the two directions of the super-audio channel are isolated.
There is a relatively high degree of interference on the super-audio channel when the audio channel is being used for dialling or signalling and the super-audio channel is thereby prevented from operating properly for data transmission.
Preferred embodiments of the invention limit the interference generated on the super-audio channel by dialling, signalling or calling signals being transmitted over the audio channel on the common two-wire link, thereby enabling the super-audio channel to be used for data transmission.
This can be done simply, efficiently and at low cost.